Polyamide molding compounds having ultrafine fillers and light-reflecting components producible therefrom

ABSTRACT

A polyamide molding compound is disclosed having a partially crystalline poly-amide, which includes partially aromatic copolyamides and classic mineral fillers and is distinguished in that the mineral filler is ultrafine chalk (CaCO 3 ) and has an average particle size of at most 100 nm. The polyamide molding compound preferably includes at most 40 weight-percent ultrafine chalk. This polyamide molding compound is preferably used for the production of reflectors and/or sub-reflectors of vehicle driving illuminators with subsequent, direct metal coating.

The present invention relates to polyamide molding compounds accordingto independent dependent claim 1 and blanks and light-reflectingcomponents producible therefrom.

Thermoplastics, from which light-reflecting components are producedthrough injection molding and subsequent metallization (vacuumdeposition, typically using aluminum), are known. Such components areheadlight reflectors for automobiles, for example. In addition to theparaboloid headlights which were previously used exclusively, two basictypes have been developed which are optimized in regard to light usageand occupied space, the projection headlight (ellipsoid, polyellipsoid)and the free-form headlight. Since the cover disks of free-formheadlights in particular may usually be designed without profilingbecause of the optimized light usage and distribution of this type ofreflector, currently transparent disks made of polycarbonate or glassare used. This increases the requirements for the surface quality ofelements which are easily visible from the outside (e.g., reflector,sub-reflector, frame), the dimensional stability in heat, the mechanicalstrength, simple processing, and low manufacturing tolerances also beingimportant.

Such headlight reflectors may also be subdivided into the actualreflector, which essentially has a paraboloid shape, and asub-reflector, which deviates more or less from the paraboloid shape.The reflector is the actual component, which reflects light in atargeted way for the desired illumination, and which is normallypositioned directly surrounding the incandescent bulb which produces thelight. In addition to light, the bulb also produces heat, so that thereflector is subjected to an operating temperature of approximately180-210° C., depending on its construction. For peak temperatures ofmore than 220° C. or if the optical requirements are not too high,experience has shown that only sheet metal is used as a reflectormaterial.

The part of the light-reflecting component which is farther away fromthe light source is called the sub-reflector. Sub-reflectors often coverthe region between the reflector and the bulb housing and/or theremaining vehicle body or even the transparent bulb covering.Sub-reflectors therefore do not have to have a paraboloid extensionwhich is used to increase the light yield, rather, they may fulfill anaesthetic object in that they represent a reflecting surface whichappears to enlarge the reflector. Because of the greater distance fromthe light source, an operating temperature of at most approximately 150°C. is to be expected for sub-reflectors.

Metal coatings which are applied to the sub-reflectors to improve thereflection on the surfaces of the reflectors and to produce an aestheticimpression are not subjected to any direct mechanical stress, such asabrasion. Nonetheless, good adhesion of the metal coating on thereflector and sub-reflector surfaces is important, since blistering oreven flaking may impair the light yield and worsen the aestheticimpression. In the following, the term “reflector” always also refers tosub-reflectors if no express differentiation is made between reflectorsand sub-reflectors.

The metallization of the reflectors is typically performed in vacuum bymeans of vapor deposition using PVD methods (PVD=physical vapordeposition, e.g., deposition or sputtering of aluminum, for example)and/or CVD methods (CVD=chemical vapor deposition, such asplasma-enhanced CVD). An important requirement for the plastic istherefore a low outgassing rate under the corresponding vacuum andtemperature conditions. In order that the metal coatings of thereflectors are not damaged in operation, no increased outgassing mayoccur even at the high operating temperatures cited. In addition, thereflectors are to be dimensionally stable in a temperature range from−50° C. to 220° C., i.e., the expansion and contraction behavior is tobe as isotropic as possible, so that—at least for the reflectors—thelight yield and/or light bundling is not impaired. The metal coatingspreferably have expansion and contraction behavior which is essentiallyidentical to that of the reflectors, so that the tensile and/or shearingload of the reflective coatings is as small as possible. In this way,the danger of cracking or buckling in the reflective coatings is alsoreduced.

A further requirement relates to the surface qualities of the (usuallycurved) plastic surface to be coated. Especially for reflectors in whichthe light yield is essential, a smooth, high-gloss surface which is ashomogeneous as possible must be provided for the coating. Plastics whichflow poorly or solidify too early and/or an addition of fillers oftenleads to a rough, matte, or irregular impression in the injection mold,measured by the extremely high requirements of a mirror-smooth surface,even if the corresponding surface of the molding tool is polished to ahigh gloss.

Until now, duroplastics, and also, more rarely, thermoplastics, wereused to produce reflectors. Of the latter, the amorphous thermoplasticsprimarily used, e.g., polyether imide (PEI) or polyether sulfones (PESand/or PSU or PPSU) have a high glass transition temperature (Tg). Theseamorphous high-Tg thermoplastics (HT thermoplastics) may be used withoutfillers to produce reflector blanks having outstanding surface gloss.The reflector blanks may be metallized directly. However, the high priceof these amorphous HT thermoplastics is disadvantageous for massproduction. The highest temperatures occur in the illumination unit, ofcourse. Therefore, until now either the reflectors were made of sheetmetal or metallized injection molded parts were produced fromduroplastic (BMC) or amorphous HT thermoplastics (PC-HT, PEI, PSU, PES).The high tolerance requirements, coupled with the surface quality of theinjection molded parts necessary for metallization, were fulfilled untilnow only by unfilled amorphous HT thermoplastics or enameledduroplastics, so that the use of partially crystalline materials wasgenerally excluded.

Through the introduction of clear glass lenses, which are overwhelminglyused on the European market in the newer vehicle models, the frames orsub-reflectors have acquired great significance, and they are typicallycompletely metallized. In addition to the basic function of the framesas a component of the main headlight for tailoring to fender and/orengine hood geometries and illumination functions, stylistic featuresare increasingly coming to the foreground. Essential requirements of theframes are (similarly to the reflectors) easy processability,outstanding surface quality, easy metallization, resistance toenvironmental influences and moisture, temperature stability, anddimensional stability. In addition to these traditional functions,further functional units, such as reflectors for turn signals, areincreasingly integrated into the frames and/or the sub-reflector. Inorder to fulfill this requirement profile, until now a wide palette,from technical plastics to polymer blends to HT thermoplastics, wasused. Examples are polyamide, polycarbonate, polysulfone (but notpolyolefins) as well as blends based on PC, but especially on PBT. HTthermoplastics are used to achieve special thermal requirements(iridescence temperature up to 212° C. for Ultrason E from BASF,Ludwigshafen, Germany), the use of which is limited for economicreasons, however. In the course of the continuing reduction ofcomplexity, increasing integration of headlight components into highlydeveloped illumination systems is occurring, which will have highermaterial requirements (J. Queisser, M. Geprägs, R. Blum and G. Ickes,Trends bei Automobilscheinwerfern [Trends in Automobile Headlights],Kunststoffe [Plastics] 3/2002, Hanser Verlag, Munich).

The partially crystalline polyphenylene sulfide (PPS), which is cited inEuropean Patent 0 332.122 for the production of headlight reflectors,for example, also has very high thermal dimensional stability. In thiscase, a production method is disclosed in which a reflector blank (usingat most 250/% carbon black to achieve increased electrical conductivity)is injection molded in a first work step. In a second work step, thereflector blank is electrostatically enameled to compensate forirregularities and to achieve a glossy surface, and in a third workstep, it is aluminized in vacuum. This method is generally consideredtoo complicated and too expensive for the mass production of reflectors,due to this additional enameling step. In addition, it is considereddisadvantageous that the addition of fillers reduces the flowability ofan injection molding compound and roughens the sur-face of the blanksproduced in this way.

Compositions are known from European Patent 0 696 304 which include (a)a first polyamide, produced from an aromatic carboxylic acid component(isophthalic acid and/or terephthalic acid) and an aliphatic diaminecomponent (hexamethylene diamine and 2-methyl-1,5-pentamethylenediamine); (b) a second aliphatic (polyamide 66, polyamide 6, orpolyamide 46) or partially aromatic polyamide, which differs from thefirst polyamide; and (c) a mineral filler (kaolin, talc, mica, orwollastonite). It is disclosed in European Patent 0 696 304 thatcorresponding compositions having a high filler component of kaolin ormica (at least 40%) may reach an HDT/A value of more than 200° C., but aglossy surface is only observed in the cases in which the compositionalso includes 10% glass fibers. However, the addition of such glassfibers also impairs the flowability of the composition during injectionmolding of molded parts and leads to an uneven surface and to lessisotropic and/or more anisotropic contraction behavior.

Compositions are known from Japanese Patent 11 279 289 and JapanesePatent 11 303 678, which include granular metallic fillers made of Al,Ni, Sn, Cu, Fe, Au, Ag, Pt, or alloys such as brass or stainless steel(but particularly preferably Al) and from which molded parts having ametal-colored surface may be produced. The metallic impression of thesurface of a corresponding molded part is decisively determined by thegrain size of the metal particles, whose useful average diameter is tobe between 10 μm and 200 μm. If possible however, the use of suchparticulate metal additives is to be dispensed with for reasons ofeasier reclamation and/or recycling of the materials in the productionof new components.

A material for producing streetlight reflectors is known under the nameMinlon® (E.I. du Pont de Nemours & Co., Wilmington, USA). The productcited is nylon 66 (PA 66) which, in addition to a heat stabilizer, alsoincludes 36-40% classic mineral materials. However, this material doesnot appear to be suitable for vehicle travel illuminators due to thesurface quality.

Film applications are known from German Patent 198 47 844, in which acrystallizable polymer is admixed with at most 1% nanoscale fillers as anucleation agent to improve the crystallization and therefore to improvethe film properties. Thus, molded parts having higher rigidity,hardness, abrasion resistance, and toughness and/or films having goodtransparency and high gloss were achieved.

The object of the present intervention is to suggest an alternativematerial, using which injection-molded reflectors may be produced havingah at least approximately equally good surface (which is suitable fordirect coating using a metal coating) and at least approximately equallygood thermal dimensional stability as using the materials known from therelated art.

This object is achieved by the features of independent claim 1.Preferred embodiments and further features result from the dependentclaims.

The material according to the present intervention is a polyamidemolding compound having a partially crystalline polyamide and a mineralfiller, the mineral filler having an ultrafine grain with an averageparticle size of at most 100 nm.

The concept of polyamide is understood to include homopolyamides,copolyamides, and mixtures of homopolyamides and/or copolyamides.

The preferred partially aromatic copolyamides are based on the monomershexamethylene diamine and aromatic dicarboxylic acids. A partiallyaromatic copolyamide based on hexamethylene diamine, and terephthalicacid and isophthalic acid in the ratio 70/30 (i.e., one corresponding toPA 6T/6I) is especially preferred. The preferred mineral filler for thepartially aromatic copolyamide is ultrafine chalk (CaCO₃), the polyamidemolding compound preferably including at most 40 weight-percent thereof.The ultrafine chalk advantageously has an average particle diameter ofat most 90 nm, preferably an average particle diameter of at most 80 nm,and especially preferably an average particle diameter of 70. nm.

Polyamide nanocomposites having good thermal dimensional stability areknown from European Patent 0 940 430. The use of this polyamidecomposition for housings or mechanical parts in electrical equipment orelectronics (e.g., switches or plugs), external or internal parts onautomobiles, and gear or bearing housings in mechanical engineering isdisclosed. No specific use for directly coated reflectors in automobilesis disclosed in this document. In addition, European Patent 0 940 430provides no information on parameters essential in this regard, such asgloss or iridescence temperature. However, blanks may be injectionmolded from the polyamide molding.-compound of the present inventionwhich, in spite of the filler component, are distinguished by a smoothsurface having high gloss in the region where the mold was polished to ahigh gloss. This is even more astounding because, in comparison to theamorphous, unfilled high-Tg thermoplastics, both the crystallizationduring the solidification of the molding compound and the filler reducethe flowability and molding precision of the molding compound. Suchblanks are especially suitable for direct metallization (e.g., using PVDmethods) and use as reflectors.

The polyamide molding compounds according to the present invention(examples 1 and 2) were produced on a 30 mm double-screw extruder ZSK 25from Werner. & Pfleiderer at temperatures between 320° C. and 340° C. Inthis case, the polyamide was dosed into the intake and the minerals weredosed separately into the intake. Ultrafine, uncoated, precipitatedcalcium carbonate having the product name “SOCAL® U1” (Solvay ChemicalsS.A.) in the form of cubical particles with an average size of 70 nm wasused as the mineral.

The following minerals, which are not according to the presentinvention, were used in comparative examples 3 to 6:

-   -   CaCO₃ type 2: natural, milled CaCO₃ having an average particle        diameter of 3 μm, a density of 2.7 g/cm³ and a pH value of 9 and        a degree of whiteness of 90% according to DIN 53163.    -   CaCO₃ type 3: precipitated CaCO₃, fine, having an average        particle diameter of 0.2 μm, a specific surface area of 11 m²/g,        a density of 2.9 g/cm³, and a pH value of 10.    -   Kaolin: calcined kaolin, treated with aminosilane, having an        average particle diameter of 1.3 μm, a density of 2.6 g/cm³ and        a pH value of 9.

The testing of the molding compounds according to the present inventionand not according to the present invention (cf. Table 1) was performedaccording to the following guidelines:

-   -   density according to ISO 11&3    -   tensile modulus of elasticity according to ISO 527    -   HDT A, B, and C according to ISO 75

To determine the surface quality of the molding compounds according tothe present invention, slabs were produced in injection molds, polishedto a high gloss, at a compound temperature of 340° C., a moldtemperature of 140° C., and an injection speed of 30 mm/sec., and theseslabs were subsequently graded visually. TABLE 1 Example ExampleComparative example 1 2 3 4 5 6 PA 6T/6I (70/30%) Weight- 70 60 100 8080 60 percent CaCO₃ type 1 Particle Weight- 30 40 (ultrafine chalk) sizepercent 70 nm CaCO₃ type 2 Weight- 20 percent CaCO₃ type 3 Weight- 20percent Kaolin Weight- 40 percent Density dry g/cm³ 1.44 1.53 1.21 1.55Tensile modulus of dry MPa 5500 6500 4050 7500 elasticity, 23° C.Tensile modulus of dry MPa 1250 600 elasticity, 150° C. HDT A (1.8 MPa)dry ° C. 140 140 130 145 HDT B (0.45 MPa) dry ° C. 240 255 HDT C (8 MPa)dry ° C. 120 120 115 115 Surface quality — Very Very Good Poor Good Poorgood good

Blanks according to the present invention, based on partiallycrystalline, partially aromatic copolyamides, are suitable, due to theirhigh thermal dimensional stability (high HDT/A value and high meltingtemperature), for use as actual reflectors in the hot region of vehicledriving illuminators, i.e., as reflectors in automobile headlights or inheadlights of other vehicles, for example. Such blanks may 10 also beconsidered for the production of reflectors for other light facilities(e.g., stationary facilities). This astounding suitability (inconsideration of the related art up to this point) is best expressed byan iridescence temperature which is preferably over 220° C. According tothe above-mentioned magazine Kunststoffe, the highest iridescencetemperature previously reported was 212° C.

If the temperature is increased in steps, the iridescence temperature isknown to characterize the value at which the reflective layer begins todisplay iridescence, which is caused by mechanical distortion betweenthe polymer background and the metal coating because of the differingthermal expansion of these materials. An iridescence temperature ofapproximately 240° C. was measured on a reflector, produced according tothe present invention, based on PA 6T/6I (70/30). In this case, thepolyamide molding compound contained 30 weight-percent ultra-fine chalkhaving an average particle size of 70 nm. Using the polyamide moldingcompounds according to the present invention, cost-effectiveachievements of the object may be made available as a replacement formore expensive materials for both reflector temperature ranges, butparticularly for the hot range of vehicle driving illuminators.

It is also to be noted that the polyamide molding compounds may alsocontain typical additives, such as stabilizers (of differing types),flame retardants, auxiliary processing materials, antistatic agents, andfurther additives, in addition to the filler according to the presentinvention. Thus, the polyamide molding compounds of all the examplescited each also contained a heat stabilizer.

Admixing of the mineral filler to the polyamide in a double-screwextruder (compounding) is preferred as the method of producing thepolyamide molding compounds. Instead of one single type of polyamide,the use of a polyamide blend is also possible. The polyamide moldingcompounds according to the present invention are preferably used for theinjection molding of reflectors (and/or sub-reflectors). To obtainespecially precise reflector surfaces, the gas injection moldingtechnique (see PLASTVERARBEITER [Plastics Processor], May 2002,published by Hüthig Verlag, D-69121 Heidelberg, for example) may be usedduring injection molding in a special version.

1. A polyamide molding compound having a partially crystallinepolyamide, which includes partially aromatic copolyamides and classicmineral fillers, characterized in that the mineral filler is ultrafinechalk (CaCO₃) and has an average particle size of at most 100 nm.
 2. Thepolyamide molding compound according to claim 1, characterized in thatit includes at most 40 weight-percent ultrafine chalk.
 3. The polyamidemolding compound according to claim 1, characterized in that theultrafine chalk has an average particle size of at most 90 nm.
 4. Thepolyamide molding compound according to claim 1, characterized in thatthe partially aromatic copolyamides are based on the monomershexamethylene diamine and aromatic dicarboxylic acids.
 5. The polyamidemolding compound according to claim 4, characterized in that thearomatic dicarboxylic acids include terephthalic acid and isophthalicacid in the ratio 70/30.
 6. A blank made of an injection-moldedpolyamide molding compound according to claim 1, characterized in thatit includes a smooth surface having a high gloss, produced by a moldingtool polished to a high gloss.
 7. A reflector for vehicle drivingilluminators, turn signals, or street lamps, and/or a sub-reflector forvehicle driving illuminators characterized in that it includes a blankaccording to claim 6 and is metallized directly.
 8. The reflector and/orsub-reflector according to claim 7, characterized in that the metalcoating is applied through PVD methods and the iridescence temperatureis at a value which is higher than 220° C.
 9. A method of producing apolyamide molding compound having a partially crystalline polyamide,which includes partially aromatic copolyamides and classic mineralfillers, characterized in that the mineral filler is ultrafine chalk(CaCO₃), has an average particle size of at most 100 nm, and is admixedto the polyamide using a double-screw extruder.
 10. The method accordingto claim 9, characterized in that the polyamide and at most 40weight-percent ultrafine chalk are each separately dosed into the intakeof the double-screw extruder.
 11. A method of using a polyamide moldingcompound according to claim 1 comprising injection molding said moldingcompound into a reflector or sub-reflector for vehicle drivingilluminators or reflectors of turn signals or street lights.
 12. The useof a polyamide method according to claim 11, characterized in that a gasinjection molding technique is used during said injection molding. 13.The polyamide molding compound of Claim 3 wherein said average particlesize is at most 80 nm.
 14. The polyamide molding compound according toclaim 2, characterized in that the ultrafine chalk has an averageparticle size of at most 70 nm.